1. Field of Invention
The invention relates generally to brake control systems and methods for a vehicle, and more specifically to brake control systems and methods for a vehicle for linearly adjusting a braking force applied to each wheel of the vehicle.
2. Description of Related Art
Japanese Patent Application Laid-Open Publication No. 4-63755 discloses one type of a brake control system for a vehicle such as an automobile. The known type of the brake control system employs a differential pressure control valve by which a wheel cylinder pressure is linearly increased or decreased such that anti-skid control is performed.
In the aforementioned brake control system, current applied to the differential pressure control valve is controlled so as to linearly increase or decrease the wheel cylinder pressure. This may eliminate noise generated during the braking control as compared with the case where the differential pressure control valve is in the form of a switching valve, and is intermittently opened and closed. Furthermore, the above-described brake control system may reduce undesirable phenomenon such as a kick-back.
However, the above-identified publication does not disclose the manner in which the brake control system performs so-called “low select control” performed when the vehicle is running on a road with a surface having a relatively low friction coefficient and the rear-left and rear-right wheels of the vehicle are anti-skid controlled. In the low select control performed during anti-skid control, one pressure mode is selected from pressure increasing, pressure-holding and pressure decreasing modes with respect to rear left and rear right wheels by giving the highest priority to the mode for decreasing the pressure, and giving the lowest priority to the mode for increasing the pressure, and a third priority to the mode for increasing the pressure. Accordingly, the above-described brake control system may be further improved in terms of the low select control such that the braking performance of the vehicle is further improved.
In a conventional brake control system that employs two switching valves for respectively increasing (holding) the pressure and decreasing the wheel cylinder pressure for each wheel so as to control the wheel cylinder pressure, the low select control is performed by selecting one of the left and right rear wheels for which the wheel cylinder pressure should be controlled with the higher priority, measuring a time period for which the switching valves for the selected wheel are opened or closed, and switching the switching valves for the other wheel, that is, the non-selected wheel, in accordance with the open/close period of the selected wheel. As a result, the change amounts of the wheel cylinder pressures for the rear-left and rear-right wheels may be made equal to each other.
However, when a linear valve is used for increasing or decreasing the wheel cylinder pressure, the fact that the response of the linear valve varies by a larger degree compared with the response of a switching valve has to be considered. Even when the linear valve for the non-selected wheel is controlled so that a slope of change in the pressure applied to the non-selected wheel via the linear valve becomes equal to a target slope of change in the pressure applied to the selected wheel, the wheel cylinder pressures of the selected and non-selected wheels are not always increased or decreased synchronously.
The aforementioned problem occurs not only in a hydraulic type brake control system using a linear valve as a control valve, but also occurs in an electric brake control system having an electric actuating device such as an electric motor for pressing a friction member such as a brake pad against a rotating member such as a brake rotor employed in a wheel owing to variations in the response of the actuating device.